Orthogonal frequency division multiple access (OFDMA), in which the subcarriers in an orthogonal frequency division multiplexing (OFDM) symbol are assigned to different communication devices for simultaneous transmission, has been proposed as a new physical layer multiple access technique and adopted by some wireless application standards such as the wireless metropolitan area network (MAN) standard (IEEE 802.16). In a non-centralized network, communication devices are not aware which subcarriers have been occupied by other communication devices in the network. Therefore, before using the shared channel, each communication device must sense the channel to see which subcarriers are occupied. This is called subcarrier sensing. Subcarrier sensing is different from the conventional sensing, which senses the availability of the whole channel. In conventional carrier sense multiple access (CSMA), a communication device senses the whole channel and uses the whole channel if it is not occupied by others. Subcarrier sensing allows multiple communication devices to share the same channel and therefore increases the channel capacity by allowing opportunistic usage. This is also called OFDM based spectrum pooling (cf. [1]), a type of cognitive radio system. Cognitive radio is a paradigm for wireless communication in which either a network or a wireless communication device changes its transmission or reception parameters to communicate efficiently avoiding interference with licensed or unlicensed communication devices. This alteration of parameters is generally based on the active monitoring of several factors in the external and internal radio environment, such as radio frequency spectrum, communication device behavior and network state.
It is desired to provide a method to sense the availability of each of the subcarriers. In other words, it is desired to provide a method to determine whether the transmission signals are present in received signals via at least one subcarrier.